introduction to c++: part 1 · 2019. 9. 9. · introduction to c++: part 1 tutorial version 0.6...

Introduction to C++: Part 1tutorial version 0.6

Research Computing Services

Getting started with the training room terminals

Log on with your BU username If you don’t have a BU username:

Username: Choose tutm1-tutm18, tutn1-tutn18

Password: on the board.

On the desktop is a link to MobaXterm. Double click to open it.

Getting started with the training room terminals

Click the + sign at the top of the window and enter:

ssh username@scc2.bu.edu

Use your SCC account if you have one. If not, see the board

for a username and password.

When prompted enter the password.

Getting started on the SCC

Load the Eclipse module:

Enter this command to create a

directory in your home folder and to

copy in tutorial files:

module load eclipse/2019-06

/scratch/intro_to_cpp.sh

Run the Eclipse software

Enter this command to start up the Eclipse development

environment.

When this window appears just click the Launch button:

eclipse &

Run the Eclipse software

When this window appears just leave it be for now.

Tutorial Outline: All 4 Parts

Part 1:

Intro to C++

Object oriented concepts

Write a first program

Part 2:

Using C++ objects

Standard Template Library

Basic debugging

Part 3:

Defining C++ classes

Look at the details of how they

work

Part 4:

Class inheritance

Virtual methods

Available C++ tools on the

SCC

Tutorial Outline: Part 1

Very brief history of C++

Definition object-oriented programming

When C++ is a good choice

The Eclipse IDE

Object-oriented concepts

First program!

Some C++ syntax

Function calls

Create a C++ class

Very brief history of C++

For details more check out A History of C++: 1979−1991

C

C++

Object-oriented programming

Object-oriented programming

(OOP) seeks to define a program in

terms of the things in the problem:

files, molecules, buildings, cars, people,

etc.

what they need to be created and used

what they can do

• Internal data:

• Balance

• Transaction record

• Name of owner

• Functions defined for this class:

• Add money

• Withdraw money

• Get balance

class Account

Account my_account ;

dollars = my_account.withdraw(10.00) ;

print(my_account.get_balance()) ;

Objects (instances of a class)

“pseudo-code”

Object-oriented programming

OOP defines classes to represent

these things.

Classes can contain data and methods

(internal functions).

Classes control access to internal data

and methods. A public interface is

used by external code when using the

class.

This is a highly effective way of

modeling real world problems inside of

a computer program.

public interface

private data and methods

“Class Car”

Characteristics of C++“Actually I made up the term ‘object-oriented’, and I can tell you I did

not have C++ in mind.”

– Alan Kay (helped invent OO programming, the Smalltalk language, and the GUI)

C++ is…

Compiled.

A separate program, the compiler, is used to turn C++ source code into a form directly

executed by the CPU.

Strongly typed and unsafe

Conversions between variable types must be made by the programmer (strong typing) but can

be circumvented when needed (unsafe)

C compatible

call C libraries directly and C code is nearly 100% valid C++ code.

Capable of very high performance

The programmer has a very large amount of control over the program execution, compilers

are high quality.

Object oriented

With support for many programming styles (procedural, functional, etc.)

No automatic memory management (mostly)

The programmer is in control of memory usage

When to choose C++

Despite its many competitors C++ has

remained popular for ~30 years and will

continue to be so in the foreseeable future.

Why?

Complex problems and programs can be

effectively implemented OOP works in the real world.

No other language quite matches C++’s

combination of performance, libraries,

expressiveness, and ability to handle

complex programs.

Choose C++ when:

Program performance matters

Dealing with large amounts of data, multiple

CPUs, complex algorithms, etc.

Programmer productivity is less important

You’ll get more code written in less time in a

languages like Python, R, Matlab, etc.

The programming language itself can help

organize your code

In C++ your objects can closely model

elements of your problem

Complex data structures can be

implemented

Access to a vast number of libraries

Your group uses it already!

“If you’re not at all interested in performance, shouldn’t you

be in the Python room down the hall?”

― Scott Meyers (author of Effective Modern C++)

Eclipse https://www.eclipse.org

In this tutorial we will use the Eclipse integrated development environment

(IDE) for writing and compiling C++

About Eclipse Started in 2001 by IBM.

The Eclipse Foundation (2004) is an independent, non-profit corporation that maintains and

promotes the Eclipse platform.

Cross-platform: supported on Mac OSX, Linux, and Windows

Supports numerous languages: C++, C, Fortran, Java, Python, and more.

A complex tool that can be used by large software teams.

IDE Advantages

Handles build process for you

Syntax highlighting and live error detection

Code completion (fills in as you type)

Creation of files via templates

Built-in debugging

Code refactoring (ex. Change a variable

name everywhere in your code)

Much higher productivity compared with

plain text editors! …once you learn how to use it.

IDEs available on the SCC

Eclipse (used here)

geany – a minimalist IDE, simple to use

Netbeans – used in past C++ tutorials.

Simpler than Eclipse but still capable.

Spyder – Python only, part of Anaconda

Emacs – The one and only.

Some Others

Xcode for Mac OSX

Visual Studio for Windows

Visual Studio Core plus plugins

Code::Blocks (cross platform)

A first program

Click Create a new C++ project

in the Eclipse window.

A first program

For a project name use hello_world

Choose a Hello World C++ Project

and the Linux GCC toolchain.

This version of Eclipse is their “IDE

for Scientific Computing” package.

Then click the Next button.

A first program

Add your name

Everything else can stay the same.

Click Next.

A first program

Last screen. Don’t change anything

here, just click Finish.

A first program Now click the Workbench button in the welcome screen to go to the newly

created project.

hello_world.cpp

has been auto-

generated.

Under the Project

menu select the

Build Project

option.

Then click the

Run button:

Behind the Scenes: The Compilation Process

Hello, World! explained The main routine – the start of every C++ program! It

returns an integer value to the operating system and (in

this case) takes arguments to allow access to command

line arguments.

The return statement returns an integer value to the

operating system after completion. 0 means “no error”. C++

programs must return an integer value.

The two characters // together indicate a comment that is

ignored by the compiler.

Hello, World! explained

loads a header file containing function and class

definitions

Loads a namespace called std.

Namespaces are used to separate sections of code

for programmer convenience. To save typing we’ll

always use this line in this tutorial.

cout is the object that writes to the stdout device, i.e. the console

window.

It is part of the C++ standard library.

Without the “using namespace std;” line this would have been called

as std::cout. It is defined in the iostream header file.

<< is the C++ insertion operator. It is used to pass characters from

the right to the object on the left.

endl is the C++ newline character.

Header Files

C++ (along with C) uses header files as to hold definitions for the compiler to use while

compiling.

A source file (file.cpp) contains the code that is compiled into an object file (file.o).

The header (file.h) is used to tell the compiler what to expect when it assembles the

program in the linking stage from the object files.

Source files and header files can refer to any number of other header files.

When compiling the linker connects all of the object (.o) files together into the

executable.

Make some changes

Let’s put the message into some variables

of type string and print some numbers.

Things to note:

Strings can be concatenated with a + operator.

No messing with null terminators or strcat() as in

C

Some string notes:

Access a string character by brackets or

function:

msg[0] “H” or msg.at(0) “H”

C++ strings are mutable – they can be

changed in place.

Re-run and check out the output.

#include <iostream>

using namespace std;

int main() {

string hello = "Hello";

string world = "world!";

string msg = hello + " " +

world ;

cout << msg << endl;

msg[0] = 'h';

cout << msg << endl;

return 0;

}

A first C++ class: string

string is not a basic type (more

on those later), it is a class.

string hello creates an

instance of a string called hello.

hello is an object. It is

initialized to contain the string

“Hello”.

A class defines some data and a

set of functions (methods) that

operate on that data.

#include <iostream>

using namespace std;

int main() {

string hello = "Hello";

string world = "world!";

string msg = hello + " " +

world ;

cout << msg << endl;

msg[0] = 'h';

cout << msg << endl;

return 0;

}

A first C++ class: string

Update the code as you see

here.

After the last character is entered

Eclipse will display a large

number of methods defined for

the msg object.

If you click or type something

else just delete and re-type the

trailing period.

#include <iostream>

using namespace std;

int main() {

string hello = "Hello";

string world = "world!";

string msg = hello + " " + world ;

cout << msg << endl;

msg[0] = 'h';

cout << msg << endl;

msg.

return 0;

}

A first C++ class: string

Start typing the word size until

it appears in the menu.

Hit the Enter key to accept it.

Now hover your mouse cursor

over the msg.size() code and a

help window will pop up.

A first C++ class: string

Tweak the code to print the number

of characters in the string, build, and

run it.

size() is a public method, usable by

code that creates the object.

The internal tracking of the size and

the storage itself is private, visible

only inside the string class source

code. cout prints integers

without any modification!

#include <iostream>

using namespace std;

int main()

{

string hello = "Hello" ;

string world = "world!" ;

string msg = hello + " " + world ;

cout << msg << endl ;

msg[0] = 'h';

cout << msg << endl ;

cout << msg.size() << endl ;

return 0;

}

Break your code.

Remove a semi-colon. Re-compile. What messages do you get from the

compiler and Eclipse?

Fix that and break something else. Capitalize string String

C++ can have elaborate error messages when compiling. Experience is

the only way to learn to interpret them!

Fix your code so it still compiles and then we’ll move on…

C++ syntax is very similar to C, Java, or C#. Here’s a few things up front and we’ll cover

more as we go along.

Curly braces are used to denote a code block (like the main() function):

Statements end with a semicolon:

Comments are marked for a single line with a // or for multilines with a pair of /* and */ :

Variables can be declared at any time in a code block.

Basic Syntax

void my_function() {

int a ;

a=1 ;

int b;

}

int a ;

a = 1 + 3 ;

// this is a comment.

/* everything in here

is a comment */

{ … some code… }

Functions are sections of code that are called from other code. Functions always have a

return argument type, a function name, and then a list of arguments separated by

commas:

A void type means the function does not return a value.

Variables are declared with a type and a name:

int add(int x, int y) {

int z = x + y ;

return z ;

}

// No arguments? Still need ()

void my_function() {

/* do something...

but a void value means the

return statement can be skipped.*/

}

// Specify the type

int x = 100;

float y;

vector<string> vec ;

// Sometimes types can be

// inferred in C++11

auto z = x;

A sampling of arithmetic operators:

Arithmetic: + - * / % ++ --

Logical: && (AND) ||(OR) !(NOT)

Comparison: == > < >= <= !=

Sometimes these can have special meanings beyond arithmetic, for

example the “+” is used to concatenate strings.

What happens when a syntax error is made? The compiler will complain and refuse to compile the file.

The error message usually directs you to the error but sometimes the error occurs before the

compiler discovers syntax errors so you hunt a little bit.

Built-in (aka primitive or intrinsic) Types

“primitive” or “intrinsic” means these types are not objects. They have no methods or internal hidden data.

Here are the most commonly used types.

Note: The exact bit ranges here are platform and compiler dependent!

Typical usage with PCs, Macs, Linux, etc. use these values

Variations from this table are found in specialized applications like embedded system processors.

Name Name Value

char unsigned char 8-bit integer

short unsigned short 16-bit integer

int unsigned int 32-bit integer

long unsigned long 64-bit integer

bool true or false

Name Value

float 32-bit floating point

double 64-bit floating point

long long 128-bit integer

long double 128-bit floating point

http://www.cplusplus.com/doc/tutorial/variables/

Need to be sure of integer sizes?

In the same spirit as using integer(kind=8) type notation in Fortran, there are type definitions that

exactly specify exactly the bits used. These were added in C++11.

These can be useful if you are planning to port code across CPU architectures (ex. Intel 64-bit

CPUs to a 32-bit ARM on an embedded board) or when doing particular types of integer math.

For a full list and description see: http://www.cplusplus.com/reference/cstdint/

Name Name Value

int8_t uint8_t 8-bit integer

int16_t uint16_t 16-bit integer

int32_t uint32_t 32-bit integer

int64_t uint64_t 64-bit integer

#include <cstdint>

Reference and Pointer Variables

Variable and object values are stored in particular locations in the computer’s memory.

Reference and pointer variables store the memory location of other variables.

Pointers are found in C. References are a C++ variation that makes pointers easier and safer to

use.

More on this topic later in the tutorial.

string hello = "Hello";

string *hello_ptr = &hello;

string &hello_ref = hello;

The object hello

occupies some

computer memory.

A pointer to the hello object string. hello_ptr

is assigned the memory address of object

hello which is accessed with the “&” syntax.

hello_ref is a reference to a string. The hello_ref

variable is assigned the memory address of object hello

automatically.

C++ is strongly typed. It will auto-convert a variable of one type to another where it can.

Conversions that don’t change value work as expected:

increasing precision (float double) or integer floating point of at least the same precision.

Loss of precision usually works fine:

64-bit double precision 32-bit single precision.

But…be careful with this, if the larger precision value is too large the result might not be what you expect!

Type Casting

short x = 1 ;

int y = x ; // OK

string z = y ; // NO

C++ allows for C-style type casting with the syntax: (new type) expression

But when using C++ it’s best to stick with deliberate type casting using the 4 different

ways that are offered…

Type Casting

double x = 1.0 ;

int y = (int) x ;

float z = (float) (x / y) ;

Type Casting

static_cast<new type>( expression )

This is exactly equivalent to the C style cast.

This identifies a cast at compile time.

This makes it clear to another programmer that you really intended a cast that

reduces precision (ex. double float) even if it would happen automatically.

~99% of all your casts in C++ will be of this type.

dynamic_cast<new type>( expression)

Special version where type casting is performed at runtime, only works on reference

or pointer type variables.

Usually created automatically by the compiler where needed, rarely done by the

programmer.

double d = 1234.56 ;

float f = static_cast<float>(d) ;

// same as

float g = (float) d ;

// same as

float h = d ;

Type Casting – rarely used versions

const_cast<new type>( expression )

Variables labeled as const can’t have their value changed.

const_cast lets the programmer remove or add const to reference or pointer type

variables.

If you need to do this, you probably want to re-think your code!

reinterpret_cast<new type>( expression )

Takes the bits in the expression and re-uses them unconverted as a new type. Also

only works on reference or pointer type variables.

Sometimes useful when reading or writing binary files or when dealing with hardware

devices like serial or USB ports.

“unsafe”: the

compiler will not

protect you here!

The programmer

must make sure

everything is

correct!

Danger!

float RectangleArea1(float L, float W) {

return L*W ;

}

float RectangleArea2(const float L, const float W) {

// L=2.0 ;

return L*W ;

}

float RectangleArea3(const float& L, const float& W) {

return L*W ;

}

void RectangleArea4(const float& L, const float& W,

float& area) {

area= L*W ;

}

Functions

Open the project “FunctionExample” in

the Part 1 Eclipse project file. Compile and run it!

Open main.cpp

4 function calls are listed.

The 1st and 2nd functions are identical in

their behavior. The values of L and W are sent to the function,

multiplied, and the product is returned.

RectangleArea2 uses const arguments The compiler will not let you modify their values in the

function.

Try it! Uncomment the line and see what happens

when you recompile.

The 3rd and 4th versions pass the

arguments by reference with an added &

The function arguments L and W

are sent as type float.

Product is computed and returned

The return type is float.

Using the Eclipse Debugger

To show how these functions work we will use the Eclipse interactive debugger to step through the program

line-by-line to follow the function calls.

Click the Debug button:

Add Breakpoints

Breakpoints tell the debugger to halt at a

particular line so that the state of the

program can be inspected.

Right-click over the line numbers, go to

Breakpoint Types and choose C/C++

Breakpoints

Double click next to the line numbers in

the functions to add breakpoints.

Click the green arrow in the toolbar to

resume the program.

The debugger will pause the

program at the first

breakpoint.

In the right hand window

you’ll see the argument

values. Click one for details.

Let’s step through this: